I use an aluminum yardstick, which let's you do pretty accurate measurements on multiple pots/kettles, without needing to use a marker on anything (I've had problems with marker wearing off and worry about toxicity of the ink). Put a gallon of water in your pot, measure how high it is, and write it down somewhere. Then you'll always be able calculate what the measurement on the yardstick is equal to. If you keep the measurement conversions for each pot written down, you can use the yardstick for lots of different pots. The aluminum washes off easily and can be sanitized, so you can use it post-boil. I think it was $3.50 at Home Depot. I was looking for stainless steel yardstick, but couldn't find it without a cork backing, but the aluminum is fine - lots of people brew with aluminum and it's only in contact with the wort for a second.

If you want to do a DIY project you can also put a hole in your pot and get a weldless sight glass kit, but that seems like a lot of extra work to me plus more stuff to clean.

Agreed, pir squaredh will work too of course, but you will need to know the radius (or diameter) of your kettle in addition to the height of the liquid for this, instead of knowing the height-to-volume conversion I mentioned in my answer :)
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paulFeb 27 '13 at 23:46

The formula only works when the kettle is uniformly shaped throughout its height. My converted kegs have an inverted dome at the bottom. The error introduced may only be a few ounces or 100-200 ml, and may not matter for rough estimates.
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jalynn2Mar 14 '13 at 13:27

@jalynn2 Yes, this answer is for cylindrical kettles. Volume conversions for keggles are more complicated. To be really precise with a keggle you would need to know what the height to volume ratio is at multiple levels throughout the kettle, because there are also ribs on the sides of the keg that hold additional liquid, in addition to the domed bottom. There are various posts you can google for for help with the volume of a keggle, but there are different keg sizes and brands (budweiser, miller, sabco, etc.).
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paulMar 14 '13 at 21:47

I have aluminum HLT and BK and used a dremel to make a small mark at each gallon level. I marked with a china marker when the water was in there (adding one gallon at a time), and after I dried it out, I used the dremel. I marked in 3 columns, 120 degrees apart, so I'd be able to see at least one measurement no matter which side I was standing on. Also, I measured the distance between marks (a gallon is 17 tenths of an inch...my kettle has vertical walls,so it's the same for all marks). I have a caliper that measures in tenths of an inch and it's easy to get accurte volume measurements by using the etched marks and the caliper.

Ya agreed JMEYERING I brew in converted kegs and use the dip tube that came with it (a long stainless pipe/tube very similar to a corney keg dip tube but bigger and thicker = perfect for measurement and stirring). I poured in one gallon at a time into my keg (measure right don't just use a 1 gal jug, although you will be very close after 5 or 10 gal that "little bit off" will equal way more off. Anyways beyond the simple, I marked each gallon by dipping the tube in, duh, but pull it out very quick (metal specific) because the water doesnt "cling very well (marking in the water is ok only if you don't put anything in the water, ie. hand, as that will skew your reading). I used a hacksaw to mark each gallon then went back and marked each half gallon with an offset and smaller hash mark. I think half gallon marks are very important for the obvious reasons but quarter gallon marks get cramped and confusing, instead use the 1 gal and 1/2 gal marks to calculate 1/4- measurements during measuring. Be careful to always take your readings at the same place if you have a concaved bottom like I do and keep your measurement tool vertical when marking and measuring (easy to get off when trying to look in). So in closing I agree again with JMEYERING in using whatever you stir with to mark, and to add, indent the utensil instead of marking it. CHEERS MAN AND GOOD LUCK!

A brewing friend of mine uses an old broken wooden broom handle. I'm not a big fan of that, as the wood can't be clean or sanitized all that easy. However, when he put's it in water or wort, the liquid makes a very clear distinct line on the wood and stays there for a 5 minutes or so, it is very easily to take a reading. He has the stick marked for two different brew kettles and boiled it in water for a while before he started using it.
He never uses it after the boil for obviousness reasons.